Polyethylene foams and methods of their production

ABSTRACT

High-density polyethylene (HDPE) foams that are essentially free of residual chemical blowing agents and reaction-by-products of chemical blowing agent are provided. The HDPE foams can be either microcellular foams or conventional foams. The foams can be produced in extrusion, injection molding, and blow molding processes that utilize a physical blowing agent. Specific die designs useful for making high quality foams are described.

RELATED APPLICATIONS

[0001] This application is a continuation-in-part of PCT applicationserial no. PCT/US98/27118, filed Dec. 18, 1998, which is acontinuation-in-part of U.S. patent application Ser. No. 60/107,754filed Nov. 10, 1998, and a continuation-in-part of U.S. patentapplication Ser. No. 60/068, 173 filed Dec. 19, 1997.

FIELD OF THE INVENTION

[0002] The present invention relates generally to polymeric foamprocessing, and more particularly, to polyethylene foams and methods oftheir production.

BACKGROUND OF THE INVENTION

[0003] Polymeric foams include a plurality of voids, also called cells,in a polymer matrix. By replacing solid plastic with voids, polymericfoams use less raw material than solid plastics for a given volume.Thus, by using polymeric foams in many applications instead of solidplastics, material costs are reduced.

[0004] Microcellular foams generally, have smaller cell sizes and highercell densities than conventional polymeric foams. Typically,microcellular foams are defined as having average cell sizes of lessthan 100 microns and a cell density of greater than 10⁶ cells/cm³ ofsolid plastic. In a typical continuous process for forming microcellularfoam (e.g. extrusion), the pressure on a single-phase solution ofblowing agent and polymer is rapidly dropped to nucleate the cells. Thenucleation rate must be high enough to form the microcellular structure.

[0005] Several patents describe aspects of microcellular materials andmicrocellular processes. U.S. Pat. No. 4,473,665 (Martini-Vvedensky, etal.; Sep. 25, 1984) describes a process for making foamed polymer havingcells less than about 100 microns in diameter. In a technique ofMartini-Vvedensky, et al., a material precursor is saturated with ablowing agent, the material is placed under high pressure, and thepressure is rapidly dropped to nucleate the blowing agent and to allowthe formation of cells. The material then is frozen rapidly to maintaina desired distribution of microcells.

[0006] U.S. Pat. No. 5,158,986 (Cha, et al.; Oct. 27, 1992) describesformation of microcellular polymeric material using a supercriticalfluid as a blowing agent. In a batch process of Cha, et al., a plasticarticle is submerged at pressure in supercritical fluid for a period oftime, and then quickly returned to ambient conditions creating asolubility change and nucleation. In a continuous process, a polymericsheet is extruded, and then can be run through rollers in a container ofsupercritical fluid at high pressure, and then exposed quickly toambient conditions. In another continuous process, a supercriticalfluid-saturated molten polymeric stream is established. The polymericstream is rapidly heated, and the resulting thermodynamic instability(solubility change) creates sites of nucleation, while the system ismaintained under pressure preventing significant growth of cells. Thematerial then is injected into a mold cavity where pressure is reducedand cells are allowed to grow.

[0007] International patent publication no. WO 98/08667 (Burnham et al.)provides methods and systems for producing microcellular material, andmicrocellular articles. In one method of Burnham et al., a fluid, singlephase solution of a precursor of foamed polymeric material and a blowingagent is continuously nucleated by dividing the stream into separateportions and separately nucleating each of the separate portions. Thedivided streams can be recombined into a single stream of nucleated,fluid polymeric material. The recombined stream may be shaped into adesired form, for example, by a shaping die. Burnham et al. alsodescribe a die for making advantageously thick microcellular articles,that includes a multiple pathway nucleation section. Other methodsdescribe the fabrication of very thin microcellular products, as well.In particular, a method for continuously extruding microcellularmaterial onto a wire, resulting in very thin essentially closed cellmicrocellular insulating coating secured to the wire, is provided. Insome of the methods, pressure drop rate is an important feature andtechniques to control this and other parameters are described.

[0008] High-density polyethylene (HDPE) has traditionally been adifficult material to process as a foam. This, in part, arises from thelow melt strength of HDPE. Processes that employ chemical blowing agentshave been developed to produce foams from high-density polyethylene.Additionally, HDPE foams have been produced by batch processes (see, forexample, U.S. Pat. No. 5,158,986). However, the applicants are unawareof extruded or injection molded foams from HDPE produced without the useof chemical blowing agents or without the addition of another polymericcomponent, for example low density polyethylene (LDPE) or linear lowdensity polyethylene (LLDPE). It is an object of the invention,therefore, to provide a non-batch process for producing HDPE foam usinga physical blowing agent.

SUMMARY OF THE INVENTION

[0009] The invention provides HDPE foams and processes for theirproduction. The HDPE foams can be formed in extrusion, injectionmolding, or blow molding processes using physical blowing agents, andthus the foams are essentially free of residual chemical blowing agentand reaction-by-products of chemical blowing agent. The HDPE foams canbe produced over a broad density range and formed into a variety ofarticles.

[0010] In one aspect, the invention provides a foam article thatincludes a matrix of polymeric material including a plurality of cells.The polymeric material consists essentially of high-density polyethyleneand is essentially free of residual chemical blowing agent andreaction-by-products of chemical blowing agent. The article has a shapeessentially identical to that of a continuous extrudate or the interiorof a mold.

[0011] In another aspect, the invention provides a method of forming afoam article. The method includes the step of conveying polymericmaterial in a downstream direction in a polymer processing apparatus.The polymeric material consists essentially of high-densitypolyethylene. The method further includes the steps of introducing aphysical blowing agent into the polymeric material in the polymerprocessing apparatus and forming a foam article from the polymericmaterial.

[0012] Among other advantages, the invention provides processes forproducing HDPE foams that use physical blowing agents instead ofchemical blowing agents. Physical blowing agents are often lessexpensive than chemical blowing agents. Finally, processes usingphysical blowing agents are more efficient and reliable, not having todepend on a chemical reaction to determine the amount of blowing agentreleased during the foaming process.

[0013] Furthermore, the invention provides an HDPE foam containing lowamounts of, or essentially free of residual chemical blowing agent andreaction-by-products of chemical blowing agents. In some cases, thepresence of residual chemical blowing agents and reaction-by-products ofchemical blowing agents in a material is detrimental and can restrictits use. The HDPE foams, in accordance to the invention, areadvantageously suitable for applications, such as food packaging, andare more easily recycled without adverse effects.

[0014] In another aspect the invention provides specific die designsuseful for making high quality polymeric foams, and particularlymicrocellular HDPE foams. The die can be provided as part of a system ofextrusion. The die includes a nucleating pathway that decreases incross-section in a downstream direction with an included angle ofgreater than 4°.

[0015] In another embodiment of this aspect of the invention a method isprovided that involves introducing a single-phase solution of polymericmaterial and blowing agent into an inlet of a polymeric foaming die,nucleating the single-phase solution in a nucleating pathway thatdecreases in a downstream direction with an included angle of greaterthan 4° to form a nucleated polymeric stream, and releasing thenucleated stream as a polymeric foam extrudate to form a nucleatedpolymeric stream.

[0016] In another embodiment a forming die according to this aspect ofthe invention includes a nucleating pathway constructed such that when asingle-phase, non-nucleated solution of polymeric material and blowingagent is introduced into the die and conveyed through the die at a flowrate of about 100 pounds per hour, nucleation of the solution occurs toform a nucleated polymeric stream that is released from the die in aperiod of time of no more than about 0.002 second after nucleation.

[0017] In another embodiment of this aspect of the invention a method isprovided that involves introducing a single-phase, non-nucleatedsolution of polymeric material and blowing agent into a polymer formingdie. Within the die the solution is nucleated to form a nucleatedpolymeric stream. The stream is released as a polymeric microcellularextrudate from an outlet of the die in a period of time of no more thanabout 0.002 second after nucleation.

[0018] Other advantages, novel features, and objects of the inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawings,which are schematic and which are not intended to be drawn to scale. Inthe figures, each identical or nearly identical component that isillustrated in various figures is represented by a single numeral. Forpurposes of clarity, not every component is labeled in every figure, noris every component of each embodiment of the invention shown whereillustration is not necessary to allow those of ordinary skill in theart to understand the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is a schematic illustration of an extrusion system.

[0020]FIG. 2 illustrates a multihole blowing agent feed orificearrangement and extrusion screw.

[0021]FIG. 3 is a schematic illustration of an injection blow moldingsystem.

[0022]FIG. 4 is a schematic illustration of a die for the injection blowmolding system of FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

[0023] Commonly owned, co-pending international patent publication no.WO 98/08667 published Mar. 5, 1998, commonly owned, co-pendinginternational patent publication no. WO 98/31521 published Jul. 23,1998, commonly owned, co-pending international patent application serialno. PCT/US98/271 18, filed Dec. 18, 1998, commonly owned, copending U.S.provisional patent application Ser No. 60/068,173 entitled“Microcellular Extrusion/Blow Molding Process and Article Made Thereby”,filed Dec. 19, 1997, commonly owned, co-pending U.S. provisional patentapplication Ser. No. 60/107,754 entitled “Microcellular Extrusion/BlowMolding Process and Article Made Thereby”, filed Nov. 10, 1998, all areincorporated by reference.

[0024] The various embodiments and aspects of the invention will bebetter understood from the following definitions. As used herein,“nucleation” defines a process by which a homogeneous, single-phasesolution of polymeric material, in which is dissolved molecules of aspecies that is a gas under ambient conditions, undergoes formations ofclusters of molecules of the species that define “nucleation sites”,from which cells will grow. That is, “nucleation” means a change from ahomogeneous, single-phase solution to a mixture in which sites ofaggregation of at least several molecules of blowing agent are formed.Nucleation defines that transitory state when gas, in solution in apolymer melt, comes out of solution to form a suspension of bubbleswithin the polymer melt. Generally this transition state is forced tooccur by changing the solubility of the polymer melt from a state ofsufficient solubility to contain a certain quantity of gas in solutionto a state of insufficient solubility to contain that same quantity ofgas in solution. Nucleation can be effected by subjecting thehomogeneous, single-phase solution to rapid thermodynamic instability,such as rapid temperature change, rapid pressure drop, or both. Rapidpressure drop can be created using a nucleating pathway, defined below.Rapid temperature change can be created using a heated portion of anextruder, a hot glycerine bath, or the like.

[0025] A “nucleating agent” is a dispersed agent, such as talc or otherfiller particles, added to a polymer and able to promote formation ofnucleation sites from a single-phase, homogeneous solution. Thus“nucleation sites” do not define locations, within a polymer, at whichnucleating agent particles reside. A “filler” is a dispersed particleadded to replace solid plastic. “Nucleated” refers to a state of a fluidpolymeric material that had contained a single-phase, homogeneoussolution including a dissolved species that is a gas under ambientconditions, following an event (typically thermodynamic instability)leading to the formation of nucleation sites. “Non-nucleated” refers toa state defined by a homogeneous, single-phase solution of polymericmaterial and dissolved species that is a gas under ambient conditions,absent nucleation sites. A “non-nucleated” material can includenucleating agent such as talc.

[0026] A “polymeric material/blowing agent mixture” can be asingle-phase, non-nucleated solution of at least the two, a nucleatedsolution of at least the two, or a mixture in which blowing agent cellshave grown.

[0027] “Essentially closed-cell” microcellular material is meant todefine material that, at a thickness of about 200 microns, contains noconnected cell pathway through the material.

[0028] “Nucleating pathway” is meant to define a pathway that forms partof microcellular polymeric foam extrusion apparatus and in which, underconditions in which the apparatus is designed to operate (typically atpressures of from about 1500 to about 30,000 psi upstream of thenucleator and at flow rates of greater than about 10 pounds polymericmaterial per hour), the pressure of a single-phase solution of polymericmaterial admixed with blowing agent in the system drops below thesaturation pressure for the particular blowing agent concentration at arate or rates facilitating rapid nucleation. A nucleating pathwaydefines, optionally with other nucleating pathways, a nucleation ornucleating region of a device of the invention. “Reinforcing agent”, asused herein, refers to auxiliary, essentially solid material constructedand arranged to add dimensional stability, or strength or toughness, tomaterial. Such agents are typified by fibrous material as described inU.S. Pat. Nos. 4,643,940 and 4,426,470. “Reinforcing agent” does not, bydefinition, necessarily include filler or other additives that are notconstructed and arranged to add dimensional stability. Those of ordinaryskill in the art can test an additive to determine whether it is areinforcing agent in connection with a particular material.

[0029] “High-density polyethylene” (HDPE), as used herein, refers topolyethylene having a density of greater than 0.94 g/cm³. “Low-densitypolyethylene” (LDPE) as used herein, refers to polyethylene having adensity of less than 0.94 g/cm³.

[0030] In one set of embodiments, foam articles that consist essentiallyof high-density polyethylene (HDPE) are formed without the use of achemical blowing agent. In such articles, HDPE is essentially the onlypolymeric component but the article includes any variety of additives asknown in the art, such as a nucleating agent (e.g. talc). Such articlesthus, are essentially free of residual chemical blowing agent andreaction-byproducts of chemical blowing agent.

[0031] In some embodiments, the HDPE foam articles are conventionalfoams having an average cell size of larger than 100 microns. In otherpreferred embodiments, the HDPE foam articles are microcellular foamshaving average cell sizes of less than 100 microns. In some embodiments,microcellular material of the invention is produced having average cellsize of less than about 50 microns. In some embodiments particularlysmall cell size is desired, and in these embodiments material of theinvention has average cell size of less than about 30 microns, morepreferably less than about 20 microns, more preferably less than about10 microns, and more preferably still less than about 5 microns. Themicrocellular material preferably has a maximum cell size of about 100microns or preferably less than about 75 microns. In embodiments whereparticularly small cell size is desired, the material can have maximumcell size of about 50 microns, more preferably about 35 microns, andmore preferably still about 25 microns. A set of embodiments includesall combinations of these noted average cell sizes and maximum cellsizes. For example, one embodiment in this set of embodiments includesmicrocellular material having an average cell size of less than about 30microns with a maximum cell size of about 50 microns, and as anotherexample an average cell size of less than about 30 microns with amaximum cell size of about 35 microns, etc. That is, microcellularmaterial designed for a variety of purposes can be produced having aparticular combination of average cell size and a maximum cell sizepreferable for that purpose. Control of cell size is described ingreater detail below. In some embodiments of this set, the HDPE foamshave essentially closed cell structures.

[0032] The articles can be produced over a broad range of densities. Incertain embodiments, the void volume is greater than 10%, in someembodiments greater than 20%, in other embodiments greater than 50%. Ina preferred set of embodiments, the article has a void volume of between10% and 40%.

[0033] The process of forming the high-density polyethylene foamarticles employs the use of a physical blowing agent, as describedfurther below. In this set of embodiments, HDPE foam articles areproduced that have a shape essentially identical to that of a continuousextrudate or a shape essentially identical to that of the interior of amold. That is, the foam articles are produced by continuous extrusion,or molding, including blow-molding. Although some cell growth can occurfollowing extrusion, or following molding, the articles retain shapesthat are closely reminiscent of the extrudate or of the mold. This is todistinguish articles that are extruded or molded in an un-foamed state,and then later foamed by, for example, saturation with blowing agent andexpansion, as occurs in typical prior art batch processes.

[0034] Referring now to FIG. 1, a system 6 for the production of HDPEfoam is illustrated schematically. An extruder 8 includes a screw 38that rotates within a barrel 32 to convey, in a downstream direction 33,polymeric material in a processing space 35 between the screw and thebarrel. Although not shown in detail, screw 38 includes feed,transition, gas injection, mixing, and metering sections. The polymericmaterial is extruded through a die 37 fluidly connected to processingspace 35 and fixed to a downstream end 36 of barrel 32. Die 37 isconfigured to form an extrudate 39 of HDPE foam in the desired shape, asdescribed further below.

[0035] Extrusion screw is operably connected, at its upstream end, to adrive motor 40 which rotates the screw within barrel 32. Positionedalong barrel 32, optionally, are temperature control units 42. Controlunits 42 can be electrical heaters, can include passageways fortemperature control fluid, and or the like. Units 42 can be used to heata stream of pelletized or fluid polymeric material within the barrel tofacilitate melting, and/or to cool the stream to control viscosity and,in some cases, blowing agent solubility. The temperature control unitscan operate differently at different locations along the barrel, thatis, to heat at one or more locations, and to cool at one or moredifferent locations. Any number of temperature control units can beprovided. Temperature control units also can be supplied to heat a dieto which the extrusion system is connected.

[0036] Barrel 32 is constructed and arranged to receive a precursor ofpolymeric material. As used herein, “precursor of polymeric material” ismeant to include all materials that are fluid, or can form a fluid andthat subsequently can harden to form HDPE foam article. Typically, theprecursor is defined by HDPE polymer pellets, and can include otherspecies such as processing aids, fillers and nucleating agents.

[0037] Introduction of the pre-polymeric precursor, typically, utilizesa standard hopper 44 for containing pelletized polymeric material to befed into the extruder barrel through orifice 46, although a precursorcan be a fluid prepolymeric material injected through an orifice andpolymerized within the barrel via, for example, auxiliary polymerizationagents. In connection with the present invention, it is important onlythat a fluid stream of polymeric material be established in the system.From hopper 44 pellets are received into the feed section of screw andconveyed in a downstream direction in polymer processing space 35 as thescrew rotates. Heat from extrusion barrel 32 and shear forces arisingfrom the rotating screw, act to soften the pellets within the transitionsection. Typically, by the end of the first metering section thesoftened pellets have been gelated, that is, welded together to form auniform fluid stream substantially free of air pockets.

[0038] Microcellular material production according to the presentinvention preferably uses a physical blowing agent, that is, an agentthat is a gas under ambient conditions. In embodiments in which aphysical blowing agent is used, along barrel 32 of extruder 30 is a port54 in fluid communication with a source 56 of a physical blowing agent.Any of a wide variety of physical blowing agents known to those ofordinary skill in the art such as hydrocarbons, chlorofluorocarbons,nitrogen, carbon dioxide, and the like, and mixtures, can be used inconnection with the invention and, according to a preferred embodiment,source 56 provides carbon dioxide, or nitrogen, or a mixture thereof asa blowing agent. Supercritical fluid blowing agents are preferred,particularly supercritical carbon dioxide and/or nitrogen. Where asupercritical fluid blowing agent is used, a single-phase solution ofpolymeric material and blowing agent is created having viscosity reducedto the extent that extrusion and blow-molding is readily accomplishedeven with material of melt flow no more than about 0.2 g/10 min. Inparticularly preferred embodiments solely carbon dioxide or nitrogen,respectively, is used. In some embodiments carbon dioxide is used incombination with other blowing agents such as nitrogen, and in otherembodiments carbon dioxide is used alone with no other blowing agentspresent. In other embodiments carbon dioxide can be used with otherblowing agents so long as the other blowing agents do not materiallyalter the blowing process. When nitrogen is used, similarly it can beused alone, in combination with another blowing agent that adds to orchanges the blowing agent properties, or in combination with anotheragent that does not materially change the blowing process.

[0039] A pressure and metering device 58 typically is provided betweenblowing agent source 56 and port 54. Device 58 can be used to meter theblowing agent so as to control the amount of the blowing agent in thepolymeric stream within the extruder to maintain a level of blowingagent at a particular level. In a preferred embodiment, device 58 metersthe mass flow rate of the blowing agent. The blowing agent is generallyless than about 15% by weight of polymeric stream and blowing agent.According to one set of embodiments, blowing agent is added in an amountof between about 1% and 15% by weight, preferably between about 3% and12% by weight, more preferably between about 5% and 10% by weight, morepreferably still between about 7% and 9% by weight, based on the weightof the polymeric stream and blowing agent. In other embodiments very lowlevels of blowing agents are suitable, for example less than about 3%,less than about 2%, or less than about 1.5% by weight blowing agent.These blowing agent levels can find use, in some instances, where anucleating agent is used.

[0040] The pressure and metering device can be connected to a controller(not shown) that also is connected to drive motor 40 and/or a drivemechanism of a gear pump (not shown) to control metering of blowingagent in relationship to flow of polymeric material to very preciselycontrol the weight percent blowing agent in the fluid polymeric mixture.

[0041] In some embodiments the method involves introducing, into fluidpolymeric material flowing at a rate of at least about 10 lbs/hr., ablowing agent that is a gas under ambient conditions and, in a period ofless than about 1 minute, creating a single-phase solution of theblowing agent fluid in the polymer. In preferred embodiments, the rateof flow of the fluid polymeric material is at least about 40 or 60lbs/hr., more preferably at least about 80 lbs/hr., and in aparticularly preferred embodiment greater than at least about 100lbs/hr., and the blowing agent fluid is added and a single-phasesolution formed within one minute with blowing agent present in thesolution in an amount of at least about 1% by weight, in some cases atleast about 3% by weight, in other cases at least about 5% by weight, inother cases at least about 7%, and in still other cases at least about10% (although, as mentioned, in another set of preferred embodimentslower levels of blowing agent are used). In these arrangements, at leastabout 2.4 lbs per hour blowing agent, preferably CO₂, is introduced intothe fluid stream and admixed therein to form a single-phase solution.The rate of introduction of blowing agent is matched with the rate offlow of polymer to achieve the optimum blowing agent concentration.

[0042] Although port 54 can be located at any of a variety of locationsalong the barrel, according to a preferred embodiment it is located justupstream from a mixing section 60 of the screw and at a location 62 ofthe screw where the screw includes unbroken flights.

[0043] Referring now to FIG. 2, a preferred embodiment of the blowingagent port is illustrated in greater detail and, in addition, two portson opposing top and bottom sides of the barrel are shown. In thispreferred embodiment, port 154 is located in the gas injection sectionof the screw at a region upstream from mixing section 60 of screw 38(including highly-broken flights) at a distance upstream of the mixingsection of no more than about 4 full flights, preferably no more thanabout 2 full flights, or no more than 1 full flight. Positioned as such,injected blowing agent is very rapidly and evenly mixed into a fluidpolymeric stream to promote production of a single-phase solution of thefoamed material precursor and the blowing agent.

[0044] Port 154, in the preferred embodiment illustrated, is amulti-hole port including a plurality of orifices 164 connecting theblowing agent source with the extruder barrel. As shown, in preferredembodiments a plurality of ports 154 are provided about the extruderbarrel at various positions radially and can be in alignmentlongitudinally with each other. For example, a plurality of ports 154can be placed at the 12 o'clock, 3 o'clock, 6 o'clock, and 9 o'clockpositions about the extruder barrel, each including multiple orifices164. In this manner, where each orifice 164 is considered a blowingagent orifice, the invention includes extrusion apparatus having atleast about 10, preferably at least about 40, more preferably at leastabout 100, more preferably at least about 300, more preferably at leastabout 500, and more preferably still at least about 700 blowing agentorifices in fluid communication with the extruder barrel, fluidlyconnecting the barrel with a source of blowing agent.

[0045] Also in preferred embodiments is an arrangement (as shown in FIG.2) in which the blowing agent orifice or orifices are positioned alongthe extruder barrel at a location where, when a preferred screw ismounted in the barrel, the orifice or orifices are adjacent full,unbroken flights 165. In this manner, as the screw rotates, each flight,passes, or “wipes” each orifice periodically. This wiping increasesrapid mixing of blowing agent and fluid foamed material precursor by, inone embodiment, essentially rapidly opening and closing each orifice byperiodically blocking each orifice, when the flight is large enoughrelative to the orifice to completely block the orifice when inalignment therewith. The result is a distribution of relativelyfinely-divided, isolated regions of blowing agent in the fluid polymericmaterial immediately upon injection and prior to any mixing. In thisarrangement, at a standard screw revolution speed of about 30 rpm, eachorifice is passed by a flight at a rate of at least about 0.5 passes persecond, more preferably at least about 1 pass per second, morepreferably at least about 1.5 passes per second, and more preferablystill at least about 2 passes per second. In preferred embodiments,orifices 154 are positioned at a distance of from about 15 to about 30barrel diameters from the beginning of the screw (at upstream end 34).

[0046] Referring again to FIG. 1, mixing section 60 of screw 38,following the gas injection section, is constructed to mix the blowingagent and polymer stream to promote formation of a single phase solutionof blowing agent and polymer. The mixing section includes unbrokenflights which break up the stream to encourage mixing. Downstream themixing section, a metering section builds pressure in thepolymer-blowing agent stream before die 37.

[0047] Die 37 includes inner passageways having shape and dimensions(die geometry) to control the shape of the extrudate. The die, in thisembodiment, can have any of a variety of configurations, as is known inthe art, to produce microcellular foam in specific forms, for example,sheets, profiles, or strands. Dies described in international patentpublication no. WO 98/08667 incorporated herein by reference can beused. Particularly preferred dies for production of HDPE foams aredescribed further below.

[0048] In addition to shaping extrudate released from such a die, thedie can also perform the function of nucleating the single-phasesolution of polymeric material and blowing agent. As the pressure in thesingle-phase solution drops as the solution flows through die internalpassageways, solubility of the blowing agent in the polymer decreases,which is the driving force for cell nucleation. The extent of pressuredrop depends upon the dimensions of the passageway. Specifically, thedimensions that effect pressure dropping include the shape of thepassageway, the length of the passageway, and the thickness of thepassageway. Under processing conditions, the pressure drop across thedie is generally greater than 1,000 psi, preferably greater than 2,000psi, and more preferably greater than 3,000 psi.

[0049] Dies of the invention can be also configured, as known in theart, to provide a pressure drop rate (dP/dt) as the single-phasesolution flows across the passageway. Pressure drop rate, which dependsupon die geometry and flow rate, also effects the cell nucleationprocess. Typically, a sufficient pressure drop rate must be induced toachieve appropriate nucleation conditions for microcellular material. Incertain cases, it is desirable to use a process that employs lowpressure drop rates. Lower pressure drop rates, generally, allow formore freedom in die construction and resulting article dimensions. Incertain embodiments, the pressure drop rate in the solution is less than1.0 GPa/s, in some embodiments less than 0.10 GPa/s, and, in someembodiments less than 0.05 GPa/s. In other embodiments, higher pressuredrop rates are utilized, for example, in the production of certain thinproducts. In some cases, the pressure drop rate is greater than 1.0GPa/s, in others greater than 5.0 GPa/s, and in others greater than 10.0GPa/s.

[0050] In another embodiment, not illustrated, the pressure drop rate isinduced in at least one nucleating pathway prior to, or within the die.Such configurations are described in co-pending international patentpublication no. WO 98/08667 published Mar. 5, 1997 and incorporatedherein by reference.

[0051] As a result of elevated temperatures, extrudate 39 that isreleased from the die is typically soft enough so that the nucleatedcells grow. As the extrudate cools in the atmosphere and becomes moresolid, cell growth is restricted. In certain embodiments, it isadvantageous to provide external cooling means to speed the cooling rateof the extrudate. For example, in these embodiments, cooling may beaccomplished by blowing air on the extrudate, contacting the extrudatewith a cool surface, or submerging the extrudate in a liquid medium.Other equipment (not illustrated) downstream of the die can be used, asrequired, for additional shaping of the extrudate into a final form.

[0052] In other embodiments, the system of FIG. 1 is modified, as knownin the art, to function as injection molding systems or blow moldingsystems. Particularly preferred injection molding systems are describedin international patent publication no. WO 98/31521, which isincorporated by reference. Generally, injection molding systems do notinclude an extrusion die, but rather include a pathway fluidly connectedto the polymer processing space through which the polymer and blowingagent solution is injected into the mold. FIG. 3 schematicallyillustrates a blow molding system 70 and particularly preferred blowmolding systems are described in U.S. patent application Ser. No.60/068,173, which is incorporated by reference. Generally, blow moldingsystems employ parison forming dies and a blow mold that receives theparison of microcellular material out of the die.

[0053] Referring to FIG. 3, a blow molding system 70 includes extruder 8fluidly connected to a blow-molding extrusion die 72 fluidly connectedto extruder 8, and a blow mold 74 positionable to receive a parison ofmicrocellular material from the outlet of the die. Blow mold 74 can be aconventional mold, and is not described in detail here except to saythat foam parisons of the invention can be blow molded without heating,thus mold 74 need not include auxiliary heating systems. That is, a foamparison of the invention, in some cases a microcellular foam parison,can be extruded and then blow molded in mold 74 without applying heat tothe parison in the mold. The invention provides, in another aspect,specific die designs that are useful for making high quality HDPE foamextrudate. Generally, the die is constructed to provide HDPE sheet ortubes of thin walls. More specifically, the die can be constructed toprovide HDPE parisons for blow molding applications. In this aspect theinvention involves the discovery that a specific range of taper anglesof a converging nucleating microcellular polymeric die provides HDPEextrudate, including parisons for blow molding, that do not strip, ortear in the extrusion process and that are more uniform in surfaceappearance.

[0054] Referring now to FIG. 4, a die 72 of the invention is illustratedschematically in cross-section and includes an annular outer die body 26surrounding an inner die body 24 which, in turn, surrounds an innermandrel 31. The die includes a fluid inlet 76, constructed and arrangedto receive a single-phase, homogeneous solution 23 of polymeric fluidand blowing agent that is a gas under ambient conditions, defined by thejunction of the outlet of extruder 30 and a sidewall entrance of thedie. Fluid inlet 76 communicates with an annular ring-like void 18between the outer die body and inner die body that is in fluidcommunication with an annular channel 20 defined as a gap between theinner die body 24 and outer die body 26. Channel 20 fluidly communicateswith an annular section 28 of the die that is of greater width than thatof channel 20. Section 28 communicates, in turn, with a narrowed annularportion 29 defining a nucleating pathway having dimensions that create arapid pressure drop facilitating nucleation of the single-phase solutionfed to the die. At its downstream end nucleating pathway 29 fluidlycommunicates with an exit 32 of the die having a gap 34. Nucleatingpathway 29 preferably decreases in cross-section in a downstreamdirection with an included angle a, as described further below.Decreasing the cross-sectional dimension of nucleating pathway 29 in adownstream direction provides particularly high pressure drop rates, asdescribed in U.S. patent application Ser. No. 08/777,709 andInternational patent application serial no. PCT/US97/15088, incorporatedby reference. Where the pathway decreases in cross-sectional dimensionin a downstream direction, a single-phase solution can be continuouslynucleated by experiencing continuously decreasing pressure withinsuccessive, continuous portions of the flowing, single-phase stream at arate which increases.

[0055] Die 72 is constructed such that inner die body 24 can moveaxially relative to outer die body 26. Inner die body 24 can move froman upstream position as illustrated in FIG. 4 to a downstream positionin which it almost fills a region indicated as 25. Thus, when inner diebody 24 is positioned in an upstream position as illustrated in FIG. 4,region 25 defines an accumulator.

[0056] In operation, a single-phase solution 23 of polymeric materialand blowing agent is fed from extruder 30 to the die 72, first intoannular ring 18, then through channel 20, accumulator 25 (to the extentthat inner die body 24 is positioned upstream) and section 28 of the dieas a single-phase, non-nucleated solution, is nucleated through a rapidpressure drop occurring at nucleating pathway 29, and is extruded atexit 32 as a parison suitable for blow molding. When it is desired touse the accumulating feature of die 72, exit 32 can be closed (describedbelow) and non-nucleated, single-phase solution 23 of polymeric materialand blowing agent can be fed from extruder 30 into accumulator 25 whileinner die body 24 moves in an upstream direction. A load can be appliedto inner die body 24 in a downstream direction, during this procedure,to maintain in accumulator 25 an essentially constant pressure thatmaintains the polymer/blowing agent solution in a non-nucleated,single-phase condition. Then, exit 32 can be opened and inner die body24 driven in a downstream direction to nucleate and extrude amicrocellular parison. This feature allows for an extruder to be runcontinuously while parison extrusion occurs periodically.

[0057] While polymeric material nucleated in nucleating pathway 29 caninclude nucleating agent in some embodiments, in other embodiments nonucleating agent is used. In either case, the pathway is constructed soas to be able to create sites of nucleation in the absence of nucleatingagent. In particular, the nucleating pathway has dimensions creating adesired pressure drop rate through the pathway. In one set ofembodiments, the pressure drop rate is relatively high, and a wide rangeof pressure drop rates are achievable. A pressure drop rate can becreated, through the pathway, of at least about 0.1 GPa/sec in moltenpolymeric material admixed homogeneously with about 6 wt % CO₂ passingthrough the pathway of a rate of about 40 pounds fluid per hour.Preferably, the dimensions create a pressure drop rate through thepathway of at least about 0.3 GPa/sec under these conditions, morepreferably at least about 1 GPa/sec, more preferably at least about 3GPa/sec, more preferably at least about 5 GPa/sec, and more preferablystill at least about 7, 10, or 15 GPa/sec. The nucleator is constructedand arranged to subject the flowing stream to a pressure drop at a ratesufficient to create sites of nucleation at a density of at least about10⁷ or, preferably, 10⁸ sights/cm³. The apparatus is constructed andarranged to continuously nucleate a fluid stream of single-phasesolution of polymeric material and blowing agent flowing at a rate of atleast 20 lbs/hour, preferably at least about 40 lbs/hour, morepreferably at least about 60 lbs/hour, more preferably at least about 80lbs/hour, and more preferably still at least about 100, 200, or 400lbs/hour.

[0058] Die 72 is constructed such that mandrel 31 can move axiallyrelative to the remainder of the die. This allows for exit 32 to beclosed, if desired, by moving mandrel 31 in an upstream direction so asto seal the inner die lip against the outer die lip.

[0059] Included angle a of die 72 defines a particular time betweeninitiation of nucleation in the die and release from the die exit, andthis timing defines another aspect of the invention. Specifically,methods are provided that involve releasing a nucleated stream as apolymeric microcellular extrudate from an outlet of a die in a period oftime of no more than about 0.002 seconds, or no more than about 0.001seconds, after nucleation, within the die, of a single-phase,non-nucleated solution of polymeric material and blowing agent.

[0060] The invention involves the discovery of a problem in theextrusion of HDPE sheet or thin profiles. It has been discovered thatunder microcellular extrusion conditions where a parallel type nucleatoris used extrudate, especially HDPE parisons for blow molding, tend tostrip during the extrusion process. Normal microcellular conditions, inthese cases, are described by using a nucleator that creates thenecessary pressure drop rate required to form small cells and usingtypical conditions of blowing agent content and melt temperature.Stripping is defined as a condition in which extrudate, upon exiting adie, tears at one or more locations along the length of the die. Thistearing interferes with the formation of a uniform extrudate, resultingin the formation of long, thin strips of microcellular material. Eachstrip is the result of the complete severing of the extrusion at each ofthe locations of the observed tear. This phenomena has been observed inHDPE and has not been observed in other materials tested, includingpolypropylene, and is thought to be caused by the highly linear natureof the HDPE molecule and the ease with which the molecules slip past oneanother under low force.

[0061] The problem of HDPE stripping is alleviated according to theinvention by using a tapered die (a die that decreases in cross sectionin a downstream direction) of a very specific included angle α. Theincluded taper angle minimum is dictated by the ability to overcome thisstripping problem and still provide a minimum pressure drop ratenecessary to make microcellular material. Angles of less than about 4degrees do not generate the required pressure drop rate formicrocellular material at commercially reasonable rates and atacceptable total pressure drops. At angles of 6 degrees and greater, asufficient pressure drop rate can be achieved and the stripping problemis completely overcome. A maximum angle exists that still makes anacceptable structure. A angles greater than about 18 degrees, themicrocellular structure tends to blow itself apart, resulting in verypoor cell structures for blow molding purposes.

[0062] In a particularly preferred embodiment, nucleating pathway 29decreases in cross section in a downstream direction with an includedangle a of greater than 4°. Preferably, included angle α is greater than6°. In one embodiment the included angle is between 4° and 18°,preferably between 4° and 8°. “Included angle”, as used herein, meansthe total angle of downstream-direction taper. For example, in anannular die in which the exterior wall tapers inwardly at 4° and theinterior wall, defined by the exterior of a mandral, has no taper, theincluded angle is 4°. In an identical situation in which the mandraltapers outwardly at 2°, the included angle would be 6°.

[0063] The result of stripping prevention was unexpected. Although notwishing to be bound by any theory, it is believed that the tapered dieworks because there is a critical location where the beginning ofnucleation occurs. This location is defined by the point at which thepressure in the polymer/blowing agent melt (single-phase, non-nucleatedsolution of polymeric material and blowing agent) is reduced below thesaturation pressure of the blowing agent in the polymer. If thislocation is too far away from the exit of the die (measured not bydistance, but by residence time or the time it takes the nucleatedpolymer to travel from the initial nucleation point to the end of thedie where release of polymer extrudate occurs), then stripping occurswhen shear forces on the growing cell acts for too long a time resultingin tearing of the melt. If the location is closer to the exit than thiscritical point, then the foaming melt does not undergo enough shear tocause tearing. In parallel nucleators, where pressure drop rate isconstant throughout the nucleation land length and pressure decreaseslinearly throughout the nucleator, the point of nucleation occurs toofar from the exit of the die. In tapered dies, where pressure drop rateincreases throughout the nucleator and pressure decreases mostly nearthe die exit, the point of nucleation is very close to the die exit. Theresult is that shearing acts over only a short period of time and nostripping occurs. The proposed theory compels selection of a specifictaper angle that both eliminates stripping and allows for the standardconditions of pressure drop rate to be met.

[0064] Foam material produced according to the invention can be used inblow molding processes, for example in production of blow moldedbottles. Additionally, sheets of microcellular polymeric material,including microcellular HDPE, either in flat die or annular die designs,can be made. Material made with systems of the invention also can bethermoformed. In one embodiment, the die of the invention provides theability to change parison thickness by movement of the inner pin of thedie with respect to the outer die body.

[0065] In addition to die angle and gap opening, the specific dies canalso be described by the pressure and pressure drop rate needed to makedefect free, microcellular parisons.

[0066] The function and advantage of these and other embodiments of thepresent invention will be more fully understood from the examples below.The following examples are intended to illustrate the benefits of thepresent invention, but do not exemplify the full scope of the invention.

Example 1 System for Blow Molding

[0067] A tandem extrusion line including a 2 ½ inch, 32:1 L/D singlescrew primary extruder (Akron Extruders, Canal Fulton, Ohio) and a 3inch 36:1 L/D single screw secondary extruder (Akron Extruders, CanalFulton, Ohio) was arranged in a parallel configuration. A volumetricfeeder capable of suppling up to 30 lb/hr was mounted in the feed throatof the primary extruder such that compounded talc additive pellets couldbe metered into the primary extruder. An injection system for theinjection of CO₂ into the secondary was placed at approximately 8diameters from the inlet to the secondary. The injection system included4 equally spaced circumferential, radially- positioned ports, each portincluding 176 orifices, each orifice of 0.02 inch diameter, for a totalof 704 orifices. The injection system included an air actuated controlvalve to precisely meter a mass flow rate of blowing agent at rates from0.2 to 12 lbs/hr at pressures up to 5500 psi.

[0068] The screw of the primary extruder was specially designed toprovide feeding, melting and mixing of the polymer/talc concentratefollowed by a mixing section for the dispersion of blowing agent in thepolymer. The outlet of this primary extruder was connected to the inletof the secondary extruder using a transfer pipe of about 24 inches inlength.

[0069] The secondary extruder was equipped with specially designed deepchannel, multi-flighted screw to cool the polymer and maintain thepressure profile of the microcellular material precursor, betweeninjection of blowing agent and entrance to a gear pump (LCI Corporation,Charlotte, N.C.) attached to the exit of the secondary. The gear pumpwas equipped with an integral jacket for heating/cooling and sized tooperate at a maximum output of 250 lb/hr with a rated maximum dischargepressure of 10,000 psi.

[0070] The system was equipped, at exit from the gear pump, with a dieadapter and a vertically mounted blow molding die (Magic Company, Monza,Italy). The die adapter was equipped with taps for measurement of melttemperature and pressure just prior to entry into the die. The blowmolding head included a conventional spider type flow distributionchannel and a die adjustment system that allowed movement of the dierelative to the fixed position tip providing a variety of exit gapsdepending on the chosen tooling.

[0071] A two-piece bottle mold was mounted in a fixture for the handmolding of sample bottles as a secondary process. One half of the moldwas mounted stationary in the fixture with the other half mounted onlinear slides. Quick acting clamps mounted on the stationery half of themold provided the mechanism to clap the mold shut. A short section ofsteel tubing sharpened to a point attached to a 0-50 psi regulator usinga length of flexible hose provided the blow system. Mold diameter variedfrom approximately 1 inch in the cap area to 2 to 3 inches in the bodyof the bottle. The overall cavity length of the bottle mold wasapproximately 10 inches.

Example 2 Extrusion System

[0072] A tandem extrusion line as described in Example 1 was used withthe exception that the gear pump was removed and that the die adapterwas attached directly to the exit of the secondary extruder.

Example 3 Comparative; Stripped Parison Formation

[0073] High density polyethylene (Equistar LR 5403) pellets wereintroduced into the main hopper of the extrusion line described inExample 2. The tooling attached to the blow molding head included a diewith a 1.227 exit diameter and a tip of 1.181 exit diameter and 2° taperangle. This tooling configuration provided an exit gap of 0.023 inchesand an included taper angle of 2°.

[0074] The extruder was adjusted to provide an output of approximately140 lb/hr at speeds of approximately 58 rpm on the primary and 25 rpm onthe secondary. Secondary barrel temperatures were set to maintain a melttemperature of approximately 305° F. at entrance to the die. Thevolumetric feeder was turned off and no compounded talc was added. CO₂blowing agent was injected at a nominal rate of 4.8 lb/hr resulting in a3.4% by polymer weight blowing agent in the material.

[0075] At the above conditions, the time to die exit from the point ofnucleation of the polymer was approximately 0.060 seconds. Theseconditions produced stripping of the product.

Example 4 Parison Formation

[0076] High density polyethylene (Equistar LR 5403) pellets wereintroduced into the main hopper of the extrusion line described inexample 1. The tooling attached to the blow molding head included a diewith a 0.685 exit diameter and a tip of 0.623 exit diameter and 2° taperangle. This tooling configuration provided an exit gap of 0.031 inchesand an included taper angle of 4°.

[0077] The extruder and gear pump rpm were adjusted to provide an outputof approximately 216 lb/hr at speeds of approximately 78 rpm on theprimary, 32 rpm on the secondary and 50 rpm on the gear pump. Secondarybarrel temperatures were set to maintain a melt temperature ofapproximately 315° F. at entrance to the die. The additive feeder wasset to provide an output of approximately 11 lb/hr resulting in a 2.7%by polymer weight talc in the material. CO₂ blowing agent was injectedat a nominal rate of 2.2 lb/hr resulting in a 1.0% by polymer weightblowing agent in the material.

[0078] At the above conditions, the time to die exit from the point ofnucleation of the polymer was approximately 0.002 seconds. Theseconditions produced good foam with an average cell size of approximately70 microns without stripping.

Example 5 Parison Formation

[0079] High density polyethylene (Equistar LR 5403) pellets wereintroduced into the main hopper of the extrusion line described inexample 1. The tooling attached to the blow molding head included a diewith a 0.661 exit diameter and of 4° taper angle and a tip of 0.633 exitdiameter and 2° taper angle. This tooling configuration provided an exitgap of 0.014 inches and an included taper angle of 6°.

[0080] The extruder and gear pump rpm were adjusted to provide an outputof approximately 212 lb/hr at speeds of approximately 62 rpm on theprimary, 37 rpm on the secondary and 50 rpm on the gear pump. Secondarybarrel temperatures were set to maintain a melt temperature ofapproximately 315° F. at entrance to the die. The additive feeder wasset to provide an output of approximately 11 lb/hr resulting in a 2.7%by polymer weight talc in the material. CO₂ blowing agent was injectedat a nominal rate of 3.2 lb/hr resulting in a 1.5% by polymer weightblowing agent in the material.

[0081] At the above conditions, the time to die exit from the point ofnucleation of the polymer was approximately 0.003 seconds. Theseconditions produced good foam with an average cell size of approximately19 microns without stripping.

[0082] Those skilled in the art would readily appreciate that allparameters listed herein are meant to be exemplary and that actualparameters will depend upon the specific application for which themethods and apparatus of the present invention are used. It is,therefore, to be understood that the foregoing embodiments are presentedby way of example only and that, within the scope of the appended claimsand equivalents thereto, the invention may be practiced otherwise thanas specifically described.

What is claimed is:
 1. A foam article comprising: a matrix of polymericmaterial including a plurality of cells, the polymeric materialconsisting essentially of high-density polyethylene and beingessentially free of residual chemical blowing agent andreaction-by-products of chemical blowing agent and having a shapeessentially identical to that of a continuous extrudate.
 2. A foamarticle comprising: a matrix of polymeric material including a pluralityof cells, the polymeric material consisting essentially of high-densitypolyethylene and being essentially free of residual chemical blowingagent and reaction-by-products of chemical blowing agent and having ashape essentially identical to that of the interior of a mold.
 3. Thefoam article of claim 1 having an average cell size of less than 100microns.
 4. The foam article of claim 1 having an average cell size ofless than 50 microns.
 5. The foam article of claim 1 having an averagecell size of less than 20 microns. 6.. The foam article of claim 1having a void volume of greater than 10%.
 7. The foam article of claim 1having a void volume of greater than 20%.
 8. The foam article of claim 1having a void volume of greater than 50%.
 9. The foam article of claim 1having a void volume of greater than 10% and less than 50%.
 10. The foamarticle of claim 1 having an essentially closed-cell structure.
 11. Thefoam article of claim 1, including at least about 1% by weightnucleating agent.
 12. The foam article of claim 1, wherein thenucleating agent is talc.
 13. A system for extruding foam material,comprising: a polymer forming die including an inlet at an upstream endthereof constructed and arranged to receive a foam extrudate precursor,an outlet at a downstream end thereof, defining a die gap, for releasingfoamed polymeric material, and a fluid pathway connecting the inlet withthe outlet, the fluid pathway including a nucleating pathway thatdecreases in cross section in a downstream direction with an includedangle of greater than 4°.
 14. The system of claim 13 further comprising:an extruder constructed and arranged to provide microcellular extrudateprecursor comprising a single-phase, non-nucleated solution of polymericmaterial and blowing agent to the polymer foaming die including anaccumulator positionable to receive microcellular extrudate precursorfrom the extruder and to accumulate a charge of foam extrudateprecursor.
 15. The system of claim 13, wherein the polymer forming dieincludes an accumulator constructed and arranged upstream of the die gapto periodically accumulate the microcellular extrudate precursor priorto extruding through the die gap.
 16. The system of claim 13, whereinthe nucleating pathway decreases in cross section in a downstreamdirection with an included angle of greater than 6°.
 17. The system ofclaim 13, wherein the nucleating pathway decreases in cross section in adownstream direction with an included angle of greater than 7°.
 18. Thesystem claim 13, wherein the nucleating pathway decreases in crosssection in a downstream direction with an included angle of greater than8°.
 19. The system of claim 13, wherein the nucleating pathway decreasesin cross section in a downstream direction with an included angle ofgreater than 4° and less than 18°.
 20. The system of claim 13, whereinthe nucleating pathway decreases in cross section in a downstreamdirection with an included angle of greater than 4° and less than 80°.21. The system of claim 13, wherein the nucleating pathway decreases incross section in a downstream direction with an included angle ofgreater than 4°.
 22. A system for extruding microcellular polymericmaterial, comprising: a polymer forming die including an inlet at anupstream end thereof constructed and arranged to receive microcellularextrudate precursor, an outlet at a downstream end thereof, defining adie gap, for releasing foamed polymeric material, and a fluid pathwayconnecting the inlet with the outlet, the fluid pathway including anucleating pathway constructed such that when a single-phase,non-nucleated solution of polymeric material and blowing agent isintroduced into the die and conveyed through the die at a flow rate ofabout 100 lbs/hr, nucleation of the solution occurs to form a nucleatedpolymeric stream that is released from the die in a period of time of nomore than about 0.002 second after nucleation.
 23. A method of a foamarticle comprising: conveying polymeric material in a downstreamdirection in a polymer processing apparatus, the polymeric materialconsisting essentially of high-density polyethylene; introducing aphysical blowing agent into the polymeric material in the polymerprocessing apparatus; and forming a foam article from the polymericmaterial.
 24. The method of claim 23, wherein the blowing agentcomprises carbon dioxide.
 25. The method of claim 23, wherein theblowing agent comprises nitrogen.
 26. The method of claim 23, whereinforming the foam article comprises extruding the polymeric materialthrough a die connected to a downstream end of the polymer processingapparatus.
 27. The method of claim 23, wherein forming the foam articlecomprises molding the polymeric material in an injection mold connectedto a downstream end of the polymer processing space.
 28. The method ofclaim 23, wherein forming the foam article comprises blow molding thearticle.
 29. The method of claim 23, wherein the foam article has anaverage cell size of less than 100 microns.
 30. The method of claim 23,wherein the foam article has an average cell size of less than 50microns.
 31. The method of claim 23, wherein the foam article has anaverage cell size of less than 20 microns.
 32. The method of claim 23,wherein the polymeric material is essentially free of residual chemicalblowing agents or by-product of chemical blowing agent.
 33. The methodof claim 23, wherein the foam article has a void volume of greater than10%.
 34. The method of claim 23, wherein the foam article has a voidvolume of greater than 20%.
 35. The method of claim 23, wherein the foamarticle has a void volume of greater than 50%.
 36. The method of claim23, wherein the foam article has a void volume of greater than 10% andless than 50%.
 37. A method comprising: introducing a single-phasesolution of polymeric material and blowing agent into an inlet of apolymer forming die; nucleating the single-phase solution in anucleating pathway that decreases in cross-section in a downstreamdirection with an included angle of greater than 4° to form a nucleatedpolymeric stream; and releasing the nucleated stream as a polymeric foamextrudate from an outlet of the die.
 38. The method of claim 37, whereinthe die includes a nucleating pathway that decreases in cross section ina downstream direction with an included angle of greater than 6°. 39.The method of claim 37, wherein the die includes a nucleating pathwaythat decreases in cross section in a downstream direction with anincluded angle of greater than 7°.
 40. The method of claim 37, whereinthe die includes a nucleating pathway that decreases in cross section ina downstream direction with an included angle of greater than 8°. 41.The method of claim 37, wherein the polymeric material compriseshigh-density polyethylene.
 42. The method of claim 37, whereinnucleating the single-phase solution comprises subjecting thesingle-phase solution to a pressure drop rate of at least about 0.3GPa/sec.
 43. The method of claim 37, wherein nucleating the single-phasesolution comprises subjecting the single-phase solution to a pressuredrop rate of at least about 1.0 GPa/sec.
 44. The method of claims 37,wherein the single-phase solution includes blowing agent at a level ofno more than about 3% by weight based on the weight of the solution. 45.The method of claim 37, wherein releasing the nucleated stream as apolymeric microcellular extrudate from the outlet of the die, in aperiod of time of no more than about 0.001 second after nucleation. 46.The method of claim 37, wherein releasing the nucleated stream as apolymeric microcellular extrudate from the outlet of the die, in aperiod of time of no more than about 0.002 second after nucleation. 47.The method of claim 37, further comprising varying the width of a diegap defined by the die outlet, while releasing polymeric microcellularextrudate from the outlet, while maintaining a constant nucleatingpathway gap within the die by which the solution is nucleated.
 48. Themethod of claim 37, wherein the die comprises a blow molding forming dieand the polymeric material extrudate comprises a microcellular parison.49. The method of claim 48, further comprising blow molding the parisoninto a blow-molded microcellular polymeric article.
 50. A methodcomprising: introducing a single-phase solution of polymeric materialand blowing agent into an inlet of a polymer forming die; nucleating thesingle-phase solution to form a nucleated polymeric stream; andreleasing the nucleated stream as a polymeric foam extrudate from anoutlet of the die in a period of time no more than about 0.002 secondafter nucleation.